1. Field of Invention
The present invention relates to a technique for driving a light emitting device (LED), and suitable for an active matrix organic light-emitting diode (AMOLED). In particular, the present invention is directed to a technique for driving a light emitting device, and suitable for an active matrix organic light-emitting diode (AMOLED), such that the brightness of the display will not degrade as normal degradation of the materials occurs.
2. Description of Related Art
The active matrix organic light emitting diode (AMOLED) display technology is a newly developed technology, and will be mainstream for display devices accompanying liquid crystal displays (LCDs) in the future. The major feature of the AMOLED display is the use of a thin film transistor (TFT) technique to drive the organic light emitting diode, and the driving integrated circuit (IC) is installed on the panel directly, so as to be small in volume and low in cost. The AMOLED display can be applied on a medium or small sized panel in a cellular phone, PDA, digital camera and palm game player, portable DVD player and automobile global positioning system.
The digital display is characterized by a display screen composed of multiple pixels in a matrix arrangement. In order to control individual pixels, a specific pixel is commonly selected via a scanning line and a data line, and an appropriate operating voltage is also provided, so as to display information corresponding to this pixel.
In order to create an AMOLED display, a TFT substrate and organic light-emitting diode (OLED) film are incorporated into the AMOLED display pixels. When the TFT and OLED degrade, the entire display degrades as well. One approach suggests that the design of the pixels must be geared towards compensating for the degradation of the TFT, i.e., towards compensating for the shift in the threshold voltage in order for the electric current produced by the TFT to be preserved. Judging from the current technology, however, the brightness of the OLED cannot be maintained, even if the electric currents provided by the TFT are kept constant. This is because the efficiency of the OLED itself declines with time, and it declines faster than the TFT. Therefore, according to conventional techniques, even when electric currents are kept steady by the TFT, the brightness of the AMOLED display still decays.
According to FIG. 1, the brightness of the OLED depends upon the electric current, I, supplied by the TFT substrate and the OLED's own efficiency, E:B=E J=E I/A  (1)
The power of the electric current created by the TFT substrate, however, is determined by the voltage, Vgs, provided by the data driver and the threshold voltage, Vt, of the TFT:I=k (Vgs−Vt)2  (2)
The decay of the TFT is reflected in Vt, namely, decay of the TFT results in an increase in Vt, which causes a decrease in I. Usual practice, therefore, would be to compensate for the increase in Vt or to use a constant current data driver to keep the electric current constant. However, even with constant electric current, as can be seen from formula (1) above, the display's brightness will decline with the efficiency (E), which decays with time. This is a serious problem.
A second problem occurs when the display is stationary for some time. When this happens, the area being displayed decays at a faster rate than other areas. When this occurs, different brightness levels on the display will result in residual images remaining from the previous display.
In order to overcome these problems, what is needed is a circuit and method for maintaining a constant brightness to an OLED display, which accounts for more than just the change in Vt, but also compensates for degradation in the efficiency, thus addressing and solving problems associated with conventional systems.